Gas turbine engines or ramjets used in supersonic aircraft require a diffusing inlet for reducing the velocity of the air stream below sonic prior to entering the gas turbine compressor or the ramjet combustor.
Typical supersonic inlet duct diffusers are configured within the duct so as to induce a shock train starting at a particular streamwise location within the duct and continuing for one or more duct heights, depending on the duct boundary layer characteristics. As the inlet air stream passes through the shock train, pressure, temperature, and gas density increase dramatically as the fluid decelerates from supersonic to sonic velocity. Such inlet diffusers are well known, as is the common difficulty of stabilizing the location of the shock train within the duct diffuser. Even a small perturbation in gas pressure behind the shock train can push the front upstream causing it to spill out of the inlet, thereby severely reducing mass flow within the duct and increasing drag.
As pressure perturbations are known to occur with such propulsion systems, and as it is desirable to provide as short a diffuser duct length as is practical to efficiently diffuse the inlet flow stream, it is necessary to provide stabilizing means for holding the shock wave train within the inlet diffuser duct despite the inevitable back pressure perturbations. One such means in the prior art is the provision of a perforated bleed vent in the duct wall immediately upstream of the desired shock train location so as to stabilize the position of the shock train within the duct in the event of a back pressure fluctuation.
Such bleed vents in the duct walls immediately upstream of the shock front serve a dual purpose, removing the slower moving boundary layer flow which forms adjacent the wall as the air flows through the first portion of the inlet duct, and venting subsonic air flow from behind the forwardly displaced shock front thereby accommodating displacing back pressure by reducing downstream flow in the duct behind the displaced shock train, and allowing the shock front to move downstream again to the desired streamwise location as the back pressure perturbation subsides.
Such prior art stability configurations are somewhat effective, however it is known that a perforated or bleed wall section having sufficient flow to adequately stabilize the shock wave front under the expected range of back pressure fluctuation can result in a normal bleed flow rate in excess of that required to eliminate the established slow moving boundary layer. Hence the prior art stability bleed diverts too great a portion of the inlet air flow upstream of the shock train during normal operation, reducing overall inlet efficiency
It is apparent that the back pressure perturbations in a supersonic diffuser inlet duct result in the need for a variable flow stabilizing bleed or other configuration in order to accommodate the inlet diffuser's variable requirement for stabilizing bleed flow without compromising normal operating efficiency.